Radioactivity well logging system having a pad mounted neutron source and a centralized radiation detector that provides compensation for borehole fluid density and borehole diameter variations

ABSTRACT

A well logging instrument adapted for traversing an earth borehole has a plurality of neutron sources mounted in pads which contact the wall of the earth borehole. The radiation detector is centralized in the borehole and the system provides compensation for natural gamma radiation and variations in borehole fluid density and borehole diameter.

73"15lw 3R O5-U9-72 XR 3116629172 United States Patent 1151 3,662,172Youmans May 9, 1972 RADIOACTIVITY WELL LOGGING [56] References CitedSYSTEM HAVING A PAD MOUNTED NEUTRON SOURCE AND A UNITED STATES PATENTSCENTRALIZED RADIATION 2,935,615 5/ I 960 True ..250/83.6 W

3,038,075 6/1962 Y m s... DETECTOR THAT PROVIDES 3,368,195 2/1968 P3252:COMPENSATION OR BOREHOLE 3,508,439 4 1970 Alger ..250/83.6 w

FLUID DENSITY AND BOREHOLE SIGNAL SEPARATOR "SOURCE DETECTOR PrimaryExaminer-James W. Lawrence Assistant Examiner-Morton J. FromeAttorney-Robert W. Mayer, Thomas P. Hubbard, J12, Daniel Rubin, RaymondT. Majesko, Roy L. Van Winkle, William E.

Johnson, Jr. and Eddie E. Scott CALIPER SIGNAL [57] ABSTRACT A welllogging instrument adapted for traversing an earth borehole has aplurality of neutron sources mounted in pads which contact the wall ofthe earth borehole. The radiation detector is centralized in theborehole and the system provides compensation for natural gammaradiation and variations in borehole fluid density and boreholediameter.

8 Claims, 2 Drawing Figures 34/ BOREHOLE FLUID DENSITY SELECTOR BOREHOLEFLUID DENSITY COMPENSATION CALIPER COMPENSATION UNIT PATENTEDMAY 9 I972SHEET 1 UF 2 w m l N D% DT 0 .IT 3 IA .l w 3 mm M Fm mm E .l N E S E M NW U L Lo 0 Y O C C M HT H 0 E EY C R8 RT 0 N O M R B E B O D N T L E C YD A A A R A L b m WWN E 4 K Mam S I 3 3 M 3 Y M w N M S O 9 R I 2. R T EE A P 0 m m u R L \0 mm AM u 5 C AS N R 3 E GA R l SP E s 8 2 N W S B Ms \m 7 a m SOURCE INVENTOR ARTHUR H. YOU

WAN$

ATTORNEY PAIEZI SIEUIIAY 9 me SUBTRACTION BOREHOLE FLUID DENSITY 8'BOREHOLE SIZE COMPENSATION SHEET 2 [IF 2 L w m TMG AAI NGS MT NA R M rE .s 4 N 5 w T L. M CW AS RECORDER fi TRANSMISSION \DETE'CTOR INVENTORARTHUR H. YOUMANS ATTORNEY RADIOACTIVITY WELL LOGGING SYSTEM HAVING APAD MOUNTED NEUTRON SOURCE AND A CENTRALIZED RADIATION DETECTOR TI-IATPROVIDES COMPENSATION FOR BOREI'IOLE FLUID DENSITY AND BOREIIOLEDIAMETER VARIATIONS This invention relates to radioactivity well loggingin general, and more particularly, to a system for logging the effectsof neutron bombardment that eliminates the errors due to boreholediameter variations and other variable borehole parameters such asborehole fluid density variations and tool standoff.

It is well known in the prior art to irradiate the formationssurrounding a borehole with neutrons and detect radiation from theformations resulting from the neutron bombardment. lf prompt radiationis measured the system may be referred to as neutron logging and ifdelayed radiation is measured the system is termed activation loggingbecause such radiation is caused by activated elements in theformations.

One of the most serious limitations of activation well logging systemsof the prior art stems from the fact that mea-' surements have beeninfluenced by borehole diameter variations and other borehole effects.Activation well logging instruments of the prior art have been long andrigid making it impossible for both the detector and the neutron sourceto remain in contact with the wall in uneven boreholes. On this account,activation logs such as silicon logs and aluminum logs have beenobserved to be so adversely influenced by borehole variations as to beof little practical value. This is because of the twofold influence ofborehole size on prior art activation logging systems: first, anyenlargement of hole diameter reduces the intensity of neutronirradiation because of borehole fluid absorption of the neutrons; and itreduces the efficiency with which the radiation is detected because ofthe increased distance between the formation and the detector. Thepresent invention overcomes both these difficulties by eliminating theformer and providing compensation for the latter.

ln accordance with the present invention, a well logging instrument thattraverses an earth borehole is provided which has a neutron source thatcontacts the borehole wall. The neutron source is contained in a padattached to the end of an arm that extends from the central axis of theinstrument and a means to provide a continuous caliper of the boreholediameter is included. The neutron source may consist of a plurality ofindividual sources contained within a plurality of pads each attached toindividual arms that extend from the body of the instrument. Theplurality of sources, pads, and arms centralize the instrument in theborehole and serve to attenuate the neutron flux within the boreholerelative to that directed out-- wardly. The radiation detector islocated in a trailing relationship to the neutron source and issurrounded by a rubber of plastic annulus which partially displaces theborehole mud.

Prior art activation well logging systems have also encountereddifficulties due to the effect of natural gamma radiation existing inthe formations surrounding the borehole, since the radiation detectorsenses both the natural radiation and induced activation radiation. Bothare influenced by variations in borehole diameter since the mudabsorption and radiation travel distance depend on borehole diameter.

The system of the present invention includes a second gamma radiationdetector identical to the first located ahead of the neutron sources toprovide a natural gamma ray log which can be directly subtracted fromthe activation log. In addition, this system provides means to correctthe activation measurement for the effect of borehole variation. Ananalog device applies correction based on the caliper measurement andthe borehole fluid density. The caliper signal is stored in aconventional manner and delayed to correspond to detector depth. Thisdelayed signal controls the amount of correction to be applied to themeasured activation.

In one embodiment, the compensation circuit is equipped with a boreholefluid density adjustment which is set by the operator in accordance withthe known borehole fluid density. In another embodiment, means tocontinuously measure the effect of borehole fluid density and boreholesizevariation on the activation signal is provided in the subsurfaceinstrument.

This signal may then be employed to automatically vary the magnitude ofcorrection and produce an improved activation log. The latter means mayalso be used to compensate radioactivity logs other than activationlogs.

It is to be understood that the system may also be used in other loggingsystems such as neutron logging wherein the radiation from theformations is prompt radiation.

It is therefore an object of the present invention to provide a welllogging system that reduces the inaccuracies due to borehole diametervariations.

It is a further object of the present invention to provide a welllogging system that reduces the inaccuracies clue to tool standoff.

It is still a further object of the present invention to provide a welllogging system that reduces the inaccuracies due to variation inborehole'fluid density.

It is a still further object of the present invention to provide a welllogging system including a plurality of neutron source units that remainin contact with the borehole wall during the measuring operations.

It is a still further object of the present invention to provide aninstrument that will remain centralized in the borehole during themeasuring operation.

It is still further object of the present invention to provide a welllogging system that will provide a continuous caliper of the boreholediameter.

The above and other objects and advantages of the present invention willbecome apparent from a consideration of the following detaileddescription when taken in conjunction with the accompanying drawingwherein:

FIG. I illustrates schematically a side elevation partly in crosssection of a borehole instrument representing one embodiment of theinvention;

FIG. 2 illustrates another embodiment of the invention.

Referring now to FIG. 1, there is illustrated a portion of the earth'ssurface 10 traversed by an earth borehole 1.1. A well logging instrument12 is suspended in the borehole by a logging cable 13, the cable 13being wound on a drum "14 at the earths surface. The instrument 12contains a pair of neutron sources 15 and'16. The neutron sourcesconsist of encapsulated californium -ZSZ mounted in pads adapted toslide along the walls of the borehole. While two neutron sources areshown, it is to be understood that a single neutron source or apluralityof neutron sources are contemplated. If the fonnation aroundthe borehole is homogeneous and'the borehole is circular, equally goodresults will be obtained with a single source. In other cases amultiplicity of sources is preferred in order that the formation beirradiated symmetrically. it is also to be understood that neutronsources other than californium- 252 may be used, for example,americum-beryllium, radiumberyllium, and plutonium-beryllium.Californium-2S2 is preferred because of its relatively short half life,making it less of a hazard should it be lost in a borehole.

The neutron sources 15 and 16 are mounted in pad units attached to theends of arms 17 and 18. Arms 17 and 18 keep the sources in contact withthe borehole wall as the instrument logs the borehole and aids incentralizing the instrument 12 in the borehole. The instrument is shownschematically and it is v to be understood that the arms 17 and 18 areretractable to allow the instrument to move freely through the boreholecontrolled from the surface according to systems well known in the art,thereby allowing the arms to be placed in the retracted position and theinstrument moved to the portion of the borehole to be logged. The armsare then extended and the instrument is ready for operation. Theextension and retraction means also provide the force required to holdthe pads in contact with the borehole wall during logging. Pad unitsattached to arms that extend from a central instrument are well known inthe well logging art. An example of this type of pad unit is shown inUS. Pat. No. 3,068,400 to Castel et al.

Unit 19 generates a borehole caliper signal that may be transmitted tothe surface along cable 13. The borehole caliper signal may be obtainedby a potentiometer 9 with the movable tap attached to one of the armsand the potentiometer body attached to the body of instrument 12. Thepotentiometer is shown attached between arm 18 and the body ofinstrument 12 for illustrative purposes. The position of arm 18 relativeto the body of instrument 12 determines the amount of resistance in thepotentiometer circuit, hence an electrical signal may be obtainedrepresenting a caliper of the borehole. lt is to be understood thatother means of generating a borehole caliper signal are known in thewell logging art and such other means may be used in conjunction withinstrument 12 without departing from the invention.

An activation radiation detector 20 is located in a trailing positionwith reference to the neutron sources. The distance between the sourcesand detector being sufficient to prevent radiation from the sources fromobscuring the radiation being measured. As the instrument is movedthrough the borehole detector 20 detects radiation from elements in theformations that have been activated by neutrons from sources 15 and 16and transmits an electrical signal representing this activation to thesurface by cable 13. Activation radiation detector 20 may be a Geigercounter, scintillation counter, or other state of the art detectors andit is to be understood that other logging measurements are within thescope of this invention. A rubber or plastic annulus 21 surroundsdetector 20 to displace mud encountered in the borehole and therebyreduces any contribution to the activation measurement due to drillingmud activation.

Instrument 12 also contains a detector 22, identical to the activationdetector, mounted to precede the neutron sources as the instrumenttraverses the borehole. The distance between the sources and detector 22being sufficient to prevent radiation from the sources from obscuringthe radiation detected by detector 22. This detector 22 produces asignal representing natural gamma radiation existing in the formations.Since the signal generated by activation detector 20 consists ofradiation resulting from activated elements in the formations plus thenatural radiation existing in the formations, a signal representing onlyactivation radiation may be obtained by delaying the natural gammasignal from detector 22 an appropriate length of time so as tocorrespond to the activation signal from the same borehole location andthen subtracting the natural gamma signal from the activation signal.The subtraction may be done in the subsurface instrument or ashereinafter explained, the individual signal may be transmitted to thesurface equipment and processed there.

Flexible members 23 and 24 help centralize instrument 12 in theborehole. These flexible members may be extended and retracted bymovement of elements 25 and 26 along the side of instrument 12.Alternatively the flexible members may be extended and retracted bymeans well known in the art, for example, according to the system shownin U. S. Pat. No. 3,200,251 to A. H. Youmans. It is to be understoodthat this extension and retraction may be controlled from the earth'ssurface by a suitable control system (not shown in the drawing).

The various signals from the subsurface components of instrument 12 aretransmitted along cable 13 to the earths surface. Conventional means 27,including slip rings and brushes, are provided to conduct the signalfrom cable 13 to signal separator unit 28 wherein it is divided into itscomponent parts. The component parts of the signal are respectively, theactivation signal from detector 20, the natural gamma radiation signalfrom detector 22 and the caliper signal from unit 19. The natural gammaradiation signal is transmitted to delay unit 29 wherein it is delayedan appropriate length of time so as to correspond to the activationsignal from the same depth. The length of time delay provided dependsupon the distance between detectors 20 and 22 and upon the speed atwhich the instrument 12 is being moved through the borehole.

The natural gamma radiation signal is transferred from delay 29 tosubtractor 30 and the activation signal is transferred from signalseparator 28 to subtractor 30. The resultant signal from subtractor 30represents the difference between the activation signal and the naturalgamma signal, in other words an improved activation signal with theinfluence of natural radiations existing in the formations having beengreatly reduced, if not completely eliminated.

The caliper signal is transferred from signal separator 28 to delay unit31 wherein it is delayed an appropriate length of time so as tocorrespond to the activation signal from the same depth. Althoughvarious delay circuits are known to those in the art, one could, ifdesired, use a delay line such as that illustrated on pages 28 and 29 ofDigital Computer Principles, Published by the McGraw Hill Book Co. in1962 (Library of Congress Catalog Card No. 62-13207). The caliper signalis then transferred to caliper compensation unit 32 along with theactivation signal from subtractor 30. Unit 32 may be an analog devicewherein the correction applied to the activation signal depends upon themagnitude of the caliper signal. Although various subtraction, calipercompensation units, borehole fluid density compensation and boreholefluid density selector circuits are known in the art, examples of suchcircuits are discussed at length in the U. S. Pat. No. 3,538,329 to F.J. Niven, Jr., assigned to the assignee of the present application.

The caliper compensation unit 32 is connected to borehole fluid densitycompensation unit 33 such that the known density of the borehole fluidinfluences the amount of compensation made in the activation signal. Theamount of correction applied due to caliper variations depends on theborehole fluid density. This may be controlled by adjusting a selector34. It is to be understood that unit 33 could also be controlledautomatically by a subsurface device that would continuously measure theborehole fluid density as the logging instrument traverses the borehole.

The activation signal is then transmitted to a recorder 35. Theactivation signal may be correlated with depth in the borehole byproviding a measuring sheave 36 which contacts cable 13. Sheave 13drives a transmission 37 which in turn drives the recorder 35, so thatthe information recorded is related to depth in the borehole. It is tobe understood that both the natural gamma signal and the activationsignal could be compensated for the effects of variation in boreholediameter and borehole fluid density and then the natural gamma signalsubtracted from the activation signal.

Referring now to FIG. 2, another embodiment of the present invention isillustrated. Neutron sources 38 and 39 are mounted in pads attached toarms 40 and 41 which extend from subsurface instrument 42. Arms 40 and41 serve to hold the neutron sources in contact with the borehole walland centralize the instrument 42. The arms 40 and 41 may be extended andretracted by control from the surface by suitable control means (notshown in the drawing). The arms 40 and 41 may be held in the extendedposition by spring elements 61 and 62 in order to maintain contactbetween pads 38 and 39 and the borehole wall. Contained withininstrument 42 is a detector 43 for detecting radiation resulting fromactivation in the formations. The signals from detector 43 aretransmitted to the surface equipment along cable 44. Also included inconnection with instrument 42 is a means for producing a signalrepresentative of the effect of borehole fluid density and borehole sizevariations on the activation signal. This means includes a gamma raysource 45 mounted on flexible member 47 and a detector 46. The gamma raysource 45 is selected to have a gamma ray energy spectrum similar tothat of the activation radiation to be detected and an intensity greatenough to prevent the natural gamma radiation from obscuring themeasurements. Gamma ray source 45 is positioned in contact with theborehole wall by flexible member 47 attached to the instrument 42 byconnections 48 and 50. An additional flexible member 49 helps keep theinstrument centralized in the borehole and the gamma ray source 45 incontact with the borehole wall. The flexible members may be extended andretracted by a suitable means (not shown in the drawing) such as thatshown in U. S. Pat. No. 3,200,251 to A. H. Youmans. Since the gamma raysource intensity remains constant but the absorption of the gamma raysvaries as the borehole size and/or borehole fluid density varies, thesignal produced by detector 46 will show the effects of both boreholefluid density and borehole size variations. Moreover, variations in thesignal produced by detector 46 will be substantially identical withvariations which occur in the desired activation signal, which traversessubstantially the same path through the borehole. This signal istransmitted by cable 44 to be used as a correction for the activationsignal from detector 43.

A natural gamma radiation detector 51 is provided in the mannerdescribed in connection with the embodiment of FIG. 1. The signal fromdetector 51 is transmitted up cable 44.

Conventional means 52, including slip rings and brushes, are provided toconduct the signal from cable 44 to signal separator unit 54 wherein itis divided into its component parts which are, respectively, theactivation signal from detector 43, the borehole fluid density andborehole size signal from detector 46 and the natural gamma radiationsignal from detector 51.

The natural gamma radiation signal is transmitted to the delay unit 63wherein it is delayed an appropriate amount of time so as to correspondwith the activation signal from the same depth. The length of time delayprovided depends upon the distance between detectors 51 and 43 and uponthe speed at which the instrument 42 is being moved through theborehole.

The natural gamma radiation signal is transferred from delay unit 63 tosubtractor 55 and the activation signal is transferred from signalseparator 54 to subtractor 55. The resultant signal from subtractor 55represents the difference between the activation signal and the naturalgamma signal, in other words, an improved activation signal with theinfluence of natural radiation existing in the formations having beengreatly reduced, if not completely eliminated.

The improved activation signal is transferred from subtractor 55 todelay unit 56 wherein it is delayed an appropriate amount of time so asto correspond to the borehole fluid density and borehole size signalfrom the same depth. The improved activation signal is transferred toborehole fluid density and borehole size compensation unit 57 along withthe signal from subtractor 46. Unit 57 may be an analog device whereinthe correction applied to the activation signal depends upon themagnitude of the borehole fluid density and borehole size signal fromdetector 46. The improved activation signal is then transferred to arecorder 58. The activation signal may be correlated with depth in theborehole by providing a measuring sheave 59 which contacts cable 44.Sheave 59 drives a transmission 60 which in turn drives the recorder 58,so that the information recorded is related to depth in the borehole.

It is to be understood that the borehole fluid and borehole size signalcould be used to compensate both the natural gamma radiation signal andthe activation prior to subtracting the natural gamma signal from theactivation signal.

A previously described detector 43 monitors radiation emanating from theformations surrounding the borehole. This radiation may result fromneutron bombardment of the formations by sources 38 and 39. Theradiation has been described as prompt radiation when neutron logging isbeing performed and delayed radiation when activation logging is beingperformed. Under certain circumstances it is desirable to log naturalradiation from the formations. In such circumstances the formations arenot bombarded with neutrons. This may be accomplished by removing theneutron sources or by surrounding the sources with shielding to preventneutron bombardment of the formations. Detector 43 will detect naturalradiation from the formations as instrument 42 is moved through theborehole and the means for producing a signal representing the effect ofborehole fluid density variations and borehole size variations willfunction as previously described. The natural radiation signal istransmitted to signal separator 54 along cable 44 and is processed inthe manner described in connection with the activation signal exceptthat a natural gamma signal from detector 51 is not subtracted bysubtractor 55. It may be desirable to disconnect or completely removedetector 51 when a natural radiation log is being made.

The various elements of FIGS. 1 and 2 are shown diagrammatically and itis to be understood that the associated circuits and power supplies areprovided in a conventional manner. Amplification may be included in thesurface instrument, or may be affected both on the surface and in thesubsurface instrument. It is also to be understood that the instrumenthousing will be constructed to withstand the pressures and mechanicaland thermal abuses encountered in logging a deep well and provideadequate space within it to house the necessary apparatus and permit thetransmission of radiation through it. It is to be further understoodthat other forms of transmission of signals from the subsurfaceinstrument are contemplated, for example, the signals could be digitizedin the subsurface equipment and then transmitted up the cable with thesurface processing equipment being of the digital type. Alternativelythe signals from the respective detectors and transducers in thesubsurface instrument could be recorded on magnetic tape for processingin a digital computer. The various computations, compensations andcorrections hereinbefore described could then be performed by thecomputer instead of by means of the surface instrumentation hereindisclosed.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A system for investigating characteristics of formations surroundingan earth borehole comprising:

an instrument for traversing the borehole,

means connected to said instrument that contact the borehole wall forirradiating the formations with neutrons thereby inducing radiation inthe formations,

means connected to said instrument for measuring the radiation inducedin the formations and generating a signal representative of the inducedradiation,

means connected to said instrument for centering said means formeasuring the induced radiation and for measuring the diameter of theborehole and generating a signal representing the borehole diameter, and

means utilizing said signal representing the borehole diameter forcompensating said signal representative of the induced radiation forborehole diameter variations, being further characterized as includingmeans for measuring and generating a signal representing natural gammaradiation existing in the formations and means utilizing said signalrepresenting natural gamma radiation for compensating said signalrepresentative of the induced radiation for natural gamma radiationexisting in the formations.

2. The system of claim 1 including means for compensating said signalrepresentative of the induced radiation for variations in borehole fluiddensity.

3. The system of claim 2 including means for recording the compensatedsignal representative of the induced radiation as a function of depthwithin the borehole.

4. The system of claim 3 wherein the means for irradiating theformations with neutrons includes an elongated member having one endattached to said instrument, a pad unit attached to the other end ofsaid elongated member and a neutron source held by said pad so thatneutrons from said source irradiate the formations.

5. The system of claim 4 including means surrounding said means formeasuring the radiation induced in the formations for displacingborehole fluid.

6. A system for investigating the characteristics of formationssurrounding an earth borehole comprising:

an instrument for traversing the borehole,

a plurality of elongated members each having one end pivotally attachedto said instrument,

means responsive to said compensation signal for compensating saidactivation signal for variations in borehole fluid density and boreholediameter, being further characterized as including means to measure andgenerate a signal representing natural gamma radiation existing in theformation and means to subtract said signal representing natural gammaradiation from said activation signal. 7. The system of claim 6including means to record said compensated activation signal.

8. The system of claim 7 including means to correlate said compensatedactivation signal with depth in the borehole.

l I! I! l

1. A system for investigating characteristics of formations surroundingan earth borehole comprising: an instrument for traversing the borehole,means connected to said instrument that contact the borehole wall forirradiating the formations with neutrons thereby inducing radiation inthe formations, means connected to said instrument for measuring theradiation induced in the formations and generating a signalrepresentative of the induced radiation, means connected to saidinstrument for centering said means for measuring the induced radiationand for measuring the diameter of the borehole and generating a signalrepresenting the borehole diameter, and means utilizing said signalrepresenting the borehole diameter for compensating said signalrepresentative of the induced radiation for borehole diametervariations, being further characterized as including means for measuringand generating a signal representing natural gamma radiation existing inthe formations and means utilizing said signal representing naturalgamma radiation for compensating said signal representative of theinduced radiation for natural gamma radiation existing in theformations.
 2. The system of claim 1 including means for compensatingsaid signal representative of the induced radiation for variations inborehole fluid density.
 3. The system of claim 2 including means forrecording the compensated signal representative of the induced radiationas a function of depth within the borehole.
 4. The system of claim 3wherein the means for irradiating the formations with neutrons includesan elongated member having one end attached to said instrument, a padunit attached to the other end of said elongated member and a neutronsource held by said pad so that neutrons from said source irradiate theformations.
 5. The system of claim 4 including means surrounding saidmeans for measuring the radiation induced in the formations fordisplacing borehole fluid.
 6. A system for investigating thecharacteristics of formations surrounding an earth borehole comprising:an instrument for traversing the borehole, a plurality of elongatedmembers each having one end pivotally attached to said instrument, aplurality of pad units, one attached to the other end of each elongatedmember, a plurality of neutron sources, one held by each pad unit, meansconnected to said elongated members and instrument detectors formaintaining contact between said pad units and the borehole wall therebycentralizing the instrument in the borehole, means in said instrumentfor generating a signal representative of formation activation, meansconnected to said instrument for generating a compensation signalrepresenting the effect of borehole fluid density and borehole diametervariations upon the signal representative of formation activation, andmeans responsive to said compensation signal for compensating saidactivation signal for variations in borehole fluid density and boreholediameter, being further characterized as including means to measure andgenerate a signal representing natural gamma radiation existing in theformation and means to subtract said signal representing natural gammaradiation from said activation signal.
 7. The system of claim 6including means to record said compensated activation signal.
 8. Thesystem of claim 7 including means to correlate said compensatedactivation signal with depth in the borehole.